The Ministry of Defence went to extraordinary lengths to cover up its true involvement in investigating UFOs, according to secret documents revealed under the Freedom of Information Act.

The files show that officials attempted to expunge information from documents released to the Public Records Office under the "30-year rule" that would have revealed the extent of the MoD's interest in UFO sightings.

In particular, the ministry wanted to cover up the operation of a secret unit dedicated to UFO investigations within the Defence Intelligence Staff. UFO conspiracy theorists have likened the unit, called DI55, to a sort of "Men in Black" agency for defending the Earth against invasion but the released documents show this is far from the truth. One 1995 memo from DI55 to the MoD's public "UFO desk" said: "I have several books at home that describe our supposed role of 'defender of the Earth against the alien menace' - it is light years from the truth!"

The files were made public following FOI requests by David Clarke, a lecturer in journalism at Sheffield Hallam University and his colleague Andy Roberts.

"These documents don't tell us anything about UFOs but they do show how desperate the MoD have been to conceal the interest which the intelligence services had in the subject," said Dr Clarke.

The trail begins with a request, in 1976, from a UFO enthusiast called Julian Hennessy for access to the MoD's records on UFO sightings. A note from the UFO desk to the MoD's head of security on March 23 shows that officials intended to refuse him access on the grounds that the files contain confidential information and "very little of value to a serious scientific investigator".

But the note continues: "This is not to say that the investigation is not taken seriously. The branches have their own methods - and [the public UFO desk] has no 'need to know' about them - but we are aware that DI55 for example sometimes makes extensive inquiries.

"It is undesirable that even a hint of this should become public and we are currently consulting the [Air Historical Branch] on ways of expurgating the official records against the time when they qualify for disclosure [at the Public Records Office]."

Hearing of the background to his fob off 30 years ago Mr Hennessy, who is a local magistrate, was not surprised. "Everything led me to believe there was a major cover up going on," he said."They didn't want to let the public know just how interested they were in these phenomena."

Attempts to alter the public record went on into the 90s. In a note dated April 28 1993 from DI55 to the public UFO desk the unnamed author argued the unit's involvement should be excised from records due to be released under the 30-year rule. But the cat was already out of the bag. A clerical error in 1983 had meant that the distribution list was incorrectly left on a publicly released UFO-related document, so UFO enthusiasts were already asking questions.

"Since then they have obviously been bombarded by people saying who is this DI55, what do they do, what is the extent of their involvement," said Dr Clarke.

Eventually, DI55 decided to allow its involvement to be made public. A note from DI55 to the public UFO desk on 5July 1995 said: "I see no reason for continuing to deny that the [Defence Intelligence Service] has an interest in UFOs. However, if the association is formally made public then the MoD will no doubt be pressured to state what the intelligence role/interest is. This could lead to disbelief and embarrassment since few people are likely to believe the truth that lack of funds and higher priorities have prevented any study of the thousands of reports received."

At this point someone, presumably from the public UFO desk, has scribbled "ouch!" in the margin.

"The lengths they went to to remove any mention of the Defence Intelligence Staff's central role in investigating sightings suggests they had something to hide," said Dr Clarke. "But what they were hiding was not evidence of ET visits but embarrassment at the fact they were never allowed to spend public money on investigating the subject in any depth." The full extent of DI55's involvement has subsequently been made clear by a report released to Dr Clarke in May and reported in the Guardian. That threw up a 500-page document which brought together everything the unit knew about UFOs, or Unidentified Aerial Phenomena (UAPs) as the MoD prefers, including more than 10,000 sightings. It said the existence of UAPs was "indisputable", but blamed the most vexing sighting on airborne "plasmas" formed during "more than one set of weather and electrically charged conditions", or during meteor showers.

Sighting aliens or otherwise?

August 10 1965 A man reported seeing a crimson ball fly out of the side of a hill in Warminster, Wiltshire. A fortnight later, another man photographed a UFO in the centre of Warminster. In 1994 it was claimed the photo was a hoax and the object was made from a cotton reel and a button.

Boxing Day 1980 A UFO reportedly crash landed in Rendlesham forest, Suffolk, near the Woodbridge US air force base. The incident was nicknamed Britain's Roswell in a reference to the famous UFO sighting in New Mexico in 1947. Witnesses said the craft was covered in markings similar to Egyptian hieroglyphs and aliens emerged from it. An airman later confessed the incident was a hoax.

November 28 1980 Policeman Alan Godfrey reported seeing a six-metre wide dome-like object hovering in the air in Todmorden, West Yorkshire. He returned to the site with colleagues and they found the area where the object had supposedly been hovering was dry even though the rest of the road was wet because of earlier rain.

Early 1990s A string of sightings by residents in north Scotland of a UFO regularly flying overhead at great speed. Documents released earlier this year suggested the aircraft was a spy plane called Aurora, designed by the Americans to take covert pictures of the Soviet Union.

May 2006 The MoD released details of Project Condign, a four-year secret study into possible explanations for UFOs. The report concluded that many sightings could be explained as by glowing "plasmas" of gas created by charges of electricity.